Which antibiotics become toxic when expired?

Antibiotics are powerful medications used to treat bacterial infections. When used properly and not expired, antibiotics can save lives. However, antibiotics can become unsafe or less effective when they pass their expiration date. This article explores which antibiotics in particular tend to become toxic when expired.

How do antibiotics work?

Antibiotics work by killing or stopping the growth of bacteria that cause infection. Different classes of antibiotics work in different ways:

  • Penicillins like amoxicillin prevent bacteria from forming cell walls.
  • Cephalosporins like cephalexin interfere with bacterial cell walls.
  • Tetracyclines like doxycycline prevent bacterial protein synthesis.
  • Aminoglycosides like gentamicin inhibit bacterial protein synthesis.
  • Macrolides like azithromycin prevent bacterial protein synthesis.
  • Fluoroquinolones like ciprofloxacin inhibit bacterial DNA synthesis.

When antibiotics are no longer effective at these mechanisms of action, bacteria can continue to grow and the infection will not be treated.

Do antibiotics expire?

Yes, antibiotics have an expiration date printed on the bottle or box. The expiration date is the final day that the manufacturer guarantees the full potency and safety of the antibiotic.

Medications can start to lose potency before the expiration date. However, antibiotics that have passed their expiration date can become less effective or even toxic.

Why can expired antibiotics become toxic?

After the expiration date, antibiotics start to chemically degrade. The active drug chemicals start to break down into other compounds. Some degradation products can be toxic rather than therapeutic.

Some key factors that affect antibiotic degradation:

  • Temperature: Heat exposure speeds up degradation reactions.
  • Humidity: Moisture exposure can initiate chemical breakdown.
  • pH: Acidic or alkaline environments catalyze chemical transformations.
  • Oxygen: Exposure to oxygen can cause oxidative degradation.
  • Light: UV light exposure accelerates photodegradation.

Proper storage conditions like controlled room temperature help prevent degradation. Refrigeration can also extend the shelf life for some antibiotics past the expiration date.

Which antibiotics become most toxic when expired?

Tetracycline antibiotics are the most likely to become toxic when expired, especially if stored in suboptimal conditions.


Tetracycline, doxycycline, minocycline

Tetracycline antibiotics become highly nephrotoxic when expired, meaning toxic to the kidneys. Degraded tetracyclines accumulate in kidney tissue and cause damage. Kidney impairment, increased blood urea nitrogen (BUN), and renal failure can occur.


Amoxicillin, ampicillin

Expired penicillins are less likely to cause direct toxicity. However, loss of potency allows bacterial resistance and treatment failures. In severe infections like meningitis, ineffective treatment can become life-threatening.


Cephalexin, cefazolin, ceftriaxone

Similar to penicillins, degraded cephalosporins are generally not toxic but lose effectiveness. Exceptions are cephalosporins like ceftriaxone that can crystallize when old, causing damage if injected.


Ciprofloxacin, levofloxacin, moxifloxacin

Fluoroquinolones rarely become directly toxic when expired. However, they can crystallize during degradation. Crystals can block blood vessels if injected, causing tissue damage.


Azithromycin, clarithromycin

Macrolides generally degrade without toxicity. However, serious heart arrhythmias have been reported with older, degraded erythromycin.


Gentamicin, tobramycin, amikacin

Aminoglycosides can become nephrotoxic and ototoxic (toxic to the ear) when degraded, causing kidney damage and hearing loss.

Other effects of expired antibiotics

Aside from toxicity, expired antibiotics can experience other changes that reduce effectiveness:

  • Decline of potency – Less active antibiotic remains
  • Allergenicity – Inactive components may become more allergenic
  • Loss of solubility – Poor dissolving of the antibiotic in the body

Medications also often change in physical appearance when expired – discoloration, crumbling, crystallization, etc. This provides a visual cue not to use the expired antibiotic.

How to dispose of expired antibiotics

Expired or unused antibiotics should never be flushed down the drain or thrown in the trash. Antibiotic residues in waterways can promote bacterial resistance.

Instead, take leftover antibiotics to hazardous waste collection sites if available in your area. Some pharmacies also collect unused medications. If those options are not available, mix drugs with an unpalatable substance like dirt or cat litter in a sealed bag before putting in the garbage.

Be sure to never give expired antibiotics to someone else. The degraded or ineffective antibiotics could fail to treat their infection.

How to check antibiotic expiration

Always check the expiration date before taking antibiotics:

  • Look for an expiration stamped or printed on the bottle, box, foil packets, etc. This is commonly labeled as EXP followed by the date.
  • Pay attention to whether it displays a month and year, or an exact day, month, and year.
  • Do not use expired antibiotics where the date has passed.
  • Also avoid antibiotics that look discolored, crumbly, crystallized, or otherwise degraded.

Contact the pharmacy if you have questions about an antibiotic’s expiration date or condition.

How long antibiotics last past expiration

It is risky to take antibiotics past their expiration date, but some last longer than others:

Antibiotic Class Active Past Expiration
Penicillins 6-12 months when refrigerated
Cephalosporins 6-12 months when refrigerated
Tetracyclines Up to 1 year
Fluoroquinolones 2-3 years
Macrolides 2-3 years
Aminoglycosides 1-2 years

However, antibiotics stored for longer than the expiration date can become unstable, toxic, less potent, and more allergenic. Only take expired antibiotics in a true emergency and with medical guidance.

Does antibiotic effectiveness end immediately at expiration?

No, antibiotics do not necessarily stop working right at midnight of the expiration date. Loss of potency is gradual over time past the manufacturer’s guarantee:

  • 90% potency at expiration date for most antibiotics
  • 80% two years after expiration
  • 70% three years after expiration

However, degradation rates vary with storage conditions. Do not rely on potentially degraded antibiotics that are months or years expired.

Can you get sick from expired antibiotics?

In most cases, expired antibiotics will not directly cause illness, but exceptions include:

  • Allergic reactions – Allergens formed during degradation
  • Toxicity – Toxic compounds from degradation like with tetracyclines
  • Treatment failure – Bacteria not killed leading to worse infection
  • Superinfections – Resistant bacteria overgrow after weak antibiotic exposure

It is very risky to take degraded antibiotics, especially by injection. Oral antibiotics pose less immediate risk but are still dangerous when expired.


Antibiotics are generally safe when used before their expiration date and stored correctly. However, once past their expiration date, certain antibiotics like tetracyclines can degrade into toxic compounds and cause severe adverse health effects.

Expired antibiotics should never be used unless an emergency with no alternatives. When possible, always discard unused antibiotics properly and obtain a new prescription if more medication is needed. Properly used antibiotics save lives, but expired antibiotics can endanger health.

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